Exhausts from turbojet and turbofan engines are very hot and noisy. The hot exhaust plume from an engine nozzle can create a number of problems. Where the engine is so positioned relative to the airframe that the exhaust plume impinges on parts of the airframe, the hot exhaust can cause undesirable temperature-induced effects on the material properties of the impinged parts. As a result, the impinged parts may have to be constructed of a material capable of tolerating high temperatures, such as titanium, which can lead to increased cost of the airframe. In some cases, even such high-temperature materials may not be adequate to insure sufficient structural strength at the elevated temperatures caused by impingement of the exhaust gases, and thus steps must be taken to prevent impingement or to mitigate the effect of impingement.
A number of different approaches have been used in turbofan-powered aircraft in an attempt to prevent impingement or to mitigate the effect of impingement of hot core exhaust gases on airframe surfaces. One attempted solution has been to forcibly mix the hot core nozzle exhaust with relatively lower-temperature fan bypass air prior to exhausting the mixed exhaust stream out the back end of the engine, so that the resulting exhaust stream has a lower temperature. This approach requires a long, costly, and heavy bypass duct nacelle configuration in order to accommodate the mixing structure that joins and mixes the core stream with the bypass stream. A further disadvantage of this approach is that substantial losses in efficiency occur in the course of mixing the two streams, and because the streams are always mixed before being exhausted, these losses occur during all parts of an engine mission cycle, even though the exhaust gas temperature-induced problems being solved may occur during only some parts of a mission cycle such as ground and takeoff operations. Another disadvantage of this approach is that during activation of the engine fan reverser, the hot core exhaust is not mixed with fan bypass air, and thus any temperature-induced problems during fan reverser operation would not be solved.
Another attempted solution to the exhaust plume temperature problem has been to attempt to prevent impingement by use of a core exhaust thrust reverser that can be deployed when desired so as to redirect the core exhaust plume outwardly and forward. This approach has been used, for example, in cases where the hot core exhaust causes its most severe problems during activation of the engine fan thrust reverser. The core reverser hardware is costly and heavy, and requires frequent inspection and maintenance. A further shortcoming of this approach is that the core reverser is activated only during reverse-thrust operation, and thus exhaust temperature-induced problems during forward-thrust operations are not solved. Additionally, the reversed core exhaust can still impinge on airframe surfaces and cause its own temperature-induced problems. Furthermore, as with all variable-geometry reverser devices, the core exhaust thrust reverser raises safety and reliability concerns in terms of accidental deployment, failure to deploy, and/or failure to stow after deployment.